We will compare the capacity of specific serum lipoprotein fractions, from various species to inhibit cholesterol synthesis in an established line of cultured cells (hepatoma tissue culture cells (HTC cells)). Also, lipoproteins isolated from the sera of animals in different nutritional states will be tested. From the above, the most and least effective lipoproteins which affected cellular sterol synthesis will be purified, characterized, and these data will be contrasted with one another. It is hoped that the results from this study will allow us to define the properties of an exogenous effector of cholesterol synthesis. Also, we will attempt to isolate and describe cellular high affinity cholesterol receptor(s). It is proposed that one such component may play a direct role in the cascade of events which result in the regulation of sterolgenesis. The data obtained from this research proposal will compliment current studies in my laboratory which are directed at the measurement of the activity of specific, regulated sterolgenic enzymes (e.g. HMG CoA synthetase and HMG CoA reductase) in cells grown on media containing different intact or fractionated sera.